BERLIN (Reuters) - Members of Turkey's pro-Kurdish opposition party said on Tuesday they were optimistic that a majority of Turks would vote against expanding the powers of President Tayyip Erdogan in an April 16 referendum.

"Many Europeans here unfortunately think that Erdogan is invincible, but he is stoppable," Hisyar Ozsoy, a Peoples' Democratic Party (HDP) member of Turkey's parliament, said after meeting with German lawmakers.

Ozsoy, one of four HDP lawmakers visiting the German parliament this week, said he hoped that the importance of the proposed constitutional change would help boost voter turnout in Turkey and among Turks living in the diaspora in Europe.

The German government said on Tuesday it would allow 1.4 million Turks who live in Germany and are eligible to vote to cast their ballots at 13 voting sites between March 27-April 9.

Tensions between the two NATO allies have spiked since Turkey arrested a German-Turkish journalist last month, and after Erdogan accused Germany of "fascist actions" after local officials canceled some rallies in support of the referendum.

Experts say about 60 percent of Turkish voters casting polls in Germany backed Erdogan's AKP party in the last presidential election, and the vote could turn out similarly this time.

But Ozsoy cited growing concerns among Turks from across the political spectrum about the proposed changes, which he said would "turn Turkey into a dictatorship".

Turkey's parliament last month stripped one of the HDP's two leaders of her status as a member of parliament and jailed the other. The HDP, the second-largest opposition party, says 13 of its lawmakers have been jailed, and as many as 5,000 of its members have been detained.

Erdogan says the constitutional changes will give Turkey stronger leadership. But the HDP and the main opposition secular CHP say it will lead to a one-man rule and erode basic freedoms.

Erdogan and the government say the HDP is an affiliate of the militant Kurdistan Workers Party (PKK), which has waged an armed insurgency in Turkey's largely Kurdish southeast for more than three decades. The HDP denies direct links to the PKK, which is considered a terrorist organization by Turkey, the United States and the European Union.

Germany has been sharply critical of Turkey's moves to arrest 40,000 people and dismiss or suspend more than 100,000 from the military, civil service and private sector following a failed coup in July.

Sevim Dagdelen, a member of the Left party in Germany's parliament who is of Turkish background herself, said many Turkish voters in Germany would have to travel long distances to participate, which could affect turnout.

People who are not on online lists of eligible voters would also have to go through additional cumbersome steps to get registered, she said.

Kirsten Luehmann, a member of the Social Democrats in Germany, lauded Ozsoy and the other Turkish lawmakers for continuing to fight for democracy.

"They are going back to Turkey this week, despite the fact that they don't know if they will be arrested," she said. "I admire their courage to stand up for democracy in Turkey."